As electronic devices are becoming more and more portable and wireless, small and lightweight lithium ion secondary batteries with a higher energy density are gaining attention as the power source for these devices. Lithium ion secondary batteries have a positive electrode comprising a lithium-containing transition metal oxide or the like, a negative electrode comprising a carbon material or the like, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte.
For the separator of lithium ion secondary batteries, a drawn resin film (a sheet separator) is used generally. For the raw material of the resin film, polyolefins such as polyethylene and polypropylene are used. However, resin films have low heat resistance in many cases, and heat-shrinkage is caused when the film is exposed to a high temperature. Particularly under an environment of over 150° C., the shrinkage of resin film may cause deterioration of the battery safety. Particularly, when a sharp-pointed object such as a nail penetrates the battery (at the time of nail penetration test), a short circuit portion expands due to a short circuit reaction heat which is generated instantly, causing further reaction heat, raising a possibility of an acceleration of abnormal overheat.
As schematically shown in FIG. 4, there has been proposed a usage of a paste electrolyte 40 for the function of a separator. The paste electrolyte 40 includes a great amount of liquid electrolyte 41 including a thickener, and an electrically insulating filler 42. The filler 42 functions as a spacer between a positive electrode 43 and a negative electrode 44 (Japanese Laid-Open Patent Publication No. Hei 10-55718). Since the paste electrolyte is a composite material of a liquid electrolyte in which a viscosity is enhanced by the thickener and of an electrically insulating filler, liquid electrolyte is sufficiently included therein. Therefore, the paste electrolyte is excellent in that a certain level of lithium ion conductivity can be secured. However, there are defects that the strength thereof as a separator is insufficient and that the practicality is poor.
Further, there has been proposed a technology to form a porous film including a filler comprising inorganic particles on a surface of a sheet separator comprising a resin film (Japanese Laid-Open Patent Publication No. 2001-319634, and Japanese Laid-Open Patent Publication No. 2002-8730). However, since the porous film is formed on the surface of the sheet separator in these proposals, there are defects that the porous film shrinks as the sheet separator shrinks. These techniques are aiming at suppression of a lithium dendrite growth and an improvement in high-rate discharge characteristic in the first place, and are not able to ensure the safety at the time of an internal short circuit or nail penetration.
On the other hand, there has been proposed a technique in which a porous film comprising a resin with a low glass transition temperature is formed on an electrode (Japanese Laid-Open Patent Publication No. Hei 11-144706). This proposal is intended to develop the shutdown effect by softening the resin with a low glass transition temperature when a heat is generated due to a short circuit. In such proposal, for example in a nail penetration test, due to the generated heat at the time of an internal short circuit, the temperature locally exceeds several hundred ° C. depending on conditions, causing excessive softening or burning of resin. As a result, porous film deforms, and may cause an abnormal overheating. Therefore, the shutdown mechanism of the resin cannot be an absolute safety mechanism for the internal short circuit.
There has been also proposed a technique to form a protective layer comprising inorganic particles such as alumina or the like, and a water-soluble polymer on an electrode (Japanese Laid-Open Patent Publication No. Hei 9-147916). For the water-soluble polymer, a polyacrylic acid derivative, a cellulose derivative, and the like are used. Based on this proposal, since the protective layer includes inorganic particles with excellent heat resistance, suppression of the deformation of the protective layer itself at the time of heat generation due to short circuit can be expected.
However, currently, for a negative electrode of lithium ion secondary batteries, styrene-butadiene copolymer (SBR) or rubber particles comprising a modified product thereof are generally used as a negative electrode binder in many cases. This is because in the case of rubber particles, only a small usage amount is necessary, compared with the case of polyvinylidene fluoride (PVDF) conventionally used as a negative electrode binder, and lithium ion acceptability of negative electrode improves.
In the case where rubber particles are to be included in a negative electrode material mixture, usually, a thickener comprising water-soluble polymer should be used together with the rubber particles. For the water-soluble polymer, cellulose resin is the mainstream material. When a protective layer including the water-soluble polymer is applied on such negative electrode, the thickener in the negative electrode swells by water included in the protective layer before drying, causing a deformation of the negative electrode. Although the negative electrode circumvented the deformation can be applied to a practical use, the yield drops dramatically.
Also, there has been examined a separator provided by forming a thin film of paste including a resin binder dissolved in a solvent, and a filler on a surface of a positive electrode or a negative electrode, and then drying the formed film (Japanese Laid-Open Patent Publication No. Hei 10-106530). In such paste, fluorocarbon resin, polyolefin resin, or the like is included as the resin binder. Such separator is excellent in that a certain level of strength is secured. However, when the resin binder deposits on the surfaces of filler particles after dissolved in a solvent, as schematically shown in FIG. 5, the areas of filler particles 52 covered with a resin binder 51 become large. As a result, gaps among filler particles decrease while strength increases, causing an insufficient moving path between a positive electrode 53 and a negative electrode 54 for the liquid electrolyte or lithium ion to move. Additionally, although many techniques have been proposed to form a paste thin film including a resin binder dissolved in a solvent and a filler on a surface of a positive electrode or a negative electrode, the same problems will occur (Japanese Laid-Open Patent Publication No. Hei 7-220759, Patent Publication No. 3371301, Patent Publication No. 3426253).